Security vendor Bit9 promised to release limited details of a hack caused by a failure of the company to install the same security software on its own network that it sells to a handful of Fortune 500 companies.

Bit9, which is based in Waltham, Massachusetts, provides a platform that aims, among other functions, to block the installation of malicious applications. Although its product was not compromised, hackers found a weakness in company servers that issue code-signing certificates.

Once compromised, the hackers issued certificates for their own malicious software, which was then found on the networks of three Bit9 customers.

Bit9 doesn't list its customers by name on its website. But it says more than 1,000 companies use its software, including Fortune 500 companies in banking, energy, aerospace and defense and U.S. federal government agencies.

Bit9 CTO Harry Sverdlove wrote on Saturday: "We can only speculate, but we believe the attack on us was part of a larger campaign against a particular and narrow set of companies."

With Bit9's certificate, the malicious software looked at first glance as if it were legitimate and coming from the company. A valid certificate, however, isn't a free pass for malware since other kinds of security software may determine by its actions if an application is behaving in unusual ways and block it.

After news of the hack was broken by Krebs on Security, Bit9 CEO Patrick Morley disclosed the hack on Friday on a company blog. He wrote that "due to an operational oversight within Bit9, we failed to install our own product on a handful of computers within our network."

Limited information will be shared as the investigation continues, Sverdlove wrote. "For anyone who has ever been involved in an investigation of this type, you know that absolute or complete information is not always possible, so I can't promise that every puzzle piece will be revealed," he added.

Bit9 has shared cryptographic hashes, a kind of digital footprint of the files that were fraudulently signed with Bit9's certificate, Sverdlove wrote. The company plans to release more network information, tactics and files.

Hackers have proved adept at finding weaknesses in security vendors' systems with the intent to attack other targets further downstream.

In March 2011, RSA revealed hackers mounted an extremely sophisticated attack on its SecurID tokens, which generate one-time passcodes used to log into enterprise IT systems.